The present invention relates generally to the control of devices that facilitate variable cam timing during operation of an internal combustion engine such as cam phasers. The invention is particularly applicable to control of such devices during skip fire operation of an engine.
In most engines that utilize a camshaft to drive the operation of intake and exhaust valves, the camshaft is coupled to the engine crankshaft by a synchronizing mechanism such as a timing belt, a timing chain or a geared connection. Such mechanisms ensure that the timing of the opening and closing of the intake and exhaust valves are synchronized with the movements of pistons that drive the crankshaft. Traditionally, the connection between the crankshaft and camshaft was fixed, and therefore the valve timing was fixed. Although such fixed valve timing works well, it is well understood that the engine's performance and fuel efficiency can be enhanced by varying the valve timing as a function of certain current operating conditions such as engine speed and load. Therefore, over the years, a number of devices have been developed which are designed to facilitate the adjustment of the timing of the intake and/or exhaust valves during operation of an internal combustion engine. Some of these devices are arranged to adjust the rotational angle (i.e. phase) of a camshaft (and therefore any cam lobes carried thereon) relative to a crankshaft. Changing the phase of a camshaft relative to the crankshaft inherently adjusts the timing of the valves controlled by that camshaft.
One valve timing adjustment device that is popular today is known as a cam phaser. Although their designs vary, cam phasers are generally hydraulic or electric based devices. Hydraulic cam phasers tend to utilize two concentric parts with a hydraulic fluid (typically engine oil) introduced into a phaser cavity therebetween in order to control the phase of the camshaft relative to the crankshaft. The cam phaser typically includes electronically controlled hydraulic valves that direct high-pressure engine oil into the phaser cavity. Most often, a pulse width modulation (PWM) controlled solenoid is arranged to move a spool valve that regulates the flow of oil into the phaser cavity. Changing the hydraulic pressure within the phaser cavity causes a slight rotation of the camshaft relative to the cam phaser housing (and thus the crankshaft), which results in the valve timing being advanced or retarded in accordance with the rotation (phase shift) of the camshaft. A powertrain control module or engine control unit (ECU) can be arranged to adjust the camshaft timing based on factors such as the engine load and engine speed (i.e. RPM). This allows for more optimum engine performance, reduced emissions and increased fuel efficiency compared to engines with fixed camshafts. Although cam phasers work quite well for their intended purposes, they inherently introduce a certain amount of elasticity into the camshaft control.
The applicant has developed a skip fire engine control technology that is well-suited for improving the fuel efficiency of internal combustion engines. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, for example, a particular cylinder may be fired during one firing opportunity and then may be skipped during the next firing opportunity and then selectively skipped or fired during the next. This is contrasted with conventional variable displacement engine operation in which a fixed set of the cylinders are deactivated during certain low-load operating conditions.
During simulation and testing of skip fire control on vehicles that utilize cam phasers the applicant has observed that skip fire operation can cause unintended fluctuations in the cam phasing and therefore the valve timing. Such fluctuations can vary the amount of air introduced into the engine working chambers (cylinders) which can potentially negatively affect engine performance and/or fuel efficiency. Therefore, control schemes and/or devices that provide improved control over various valve timing adjustment mechanisms such as cam phasers would be desirable.